According to Smith, when people all pursue their personal interests, society benefits through the "invisible hand." This is, to Smith, much more effective than efforts to consciously improve society.
Smith believes that people, pursuing their interests, will improve society. As they pursue their interests, they will do things that make money for them. They will do things like starting businesses and pursuing profit. When they start businesses, they will eventually employ people. As they pursue profit, they make money that will be used to buy goods from other people, once again benefitting society.
All of this is better than having conscious efforts to improve society. This is because no one person or small group of people is intelligent enough to know how to improve society. It is better if everyone pursues their interest and allows the "invisible hand" of competition to improve the society.